A 2-cyanoacrylate adhesive agent has anionic polymerizability specific to 2-cyanoacrylate which is a main ingredient of 2-cyanoacrylate adhesive agent. The anionic polymerizability allows the 2-cyanoacrylate adhesive agent to start polymerization by, for example, a weak anion such as a slight amount of moisture adhered to a surface of an adherend, so that various materials can be tightly adhered to each other in a short time. For this reason, the 2-cyanoacrylate adhesive agent has been widely used as so-called instant glue in, for example, industrial, medical, and household fields.
Further, the 2-cyanoacrylate adhesive agent, which is generally transparent, has been used to adhere transparent plates or sheets, or used as an adhesive agent for optical components. Alternatively, the 2-cyanoacrylate adhesive agent is also suitably used in a field where adhesion is carried out with use of an adhesive agent in which a dye or a pigment is dissolved so as to utilize a color of the dye or the pigment. In such a field where an adhesive agent is required to be transparent, it has been reported that an adhesive whose main ingredient has a Hazen color number of 50 or more cannot be suitably used as an adhesive agent (see Patent Literature 1).
Meanwhile, 2-cyanoacrylate, which is a colorless and transparent liquid, has problems of (i) being colored to a color in a range of yellow to red due to raw materials or minute amounts of impurities produced during a production process, and (ii) being colored over time due to influence of heat, light, and humidity. In a case where 2-cyanoacrylate is colored to a degree being not less than a given level, not only in the field where an adhesive agent is required to be transparent but also in a general use, a commercial value of the 2-cyanoacrylate is significantly lost because such 2-cyanoacrylate may have to face a complaint about coloring of a liquid which is supposed to be colorless.
Given that presence of impurities is considered to be involved in coloring or acceleration of coloring of 2-cyanoacrylate, there have been presented (i) a method for producing 2-cyanoacrylate containing less acidic gas considered to be involved in production of impurities (see Patent Literature 2), and (ii) a method for preventing coloring of 2-cyanoacrylate under a generally-considered storage condition in which the 2-cyanoacrylate is stored with light shielded and at a low temperature and a low humidity. However, neither a literature nor a method has been known that presents a specific 2-cyanoacrylate-purifying method for reducing coloring of 2-cyanoacrylate which has been colored.